Direct-lit backlight having light recycling cavity with concave transflector

ABSTRACT

Direct-lit backlights and associated methods and components are disclosed in which a transflector that partially transmits and partially reflects incident light is shaped to form at least one concave structure facing a back reflector of the backlight. This provides at least one recycling cavity therebetween, the at least one recycling cavity substantially filling the output area of the backlight. At least one light source is disposed behind the output area to inject light into each cavity, and can be positioned in the recycling cavity or behind an aperture in the back reflector. The cavities are preferably shallow and wide, having a width-to-depth ratio of at least 5 or 10, and can provide uniform brightness and color at the output area with sparsely distributed light sources and in a thin profile backlight.

FIELD OF THE INVENTION

The present invention relates to backlights, particularly direct-litbacklights, as well as to components used in backlights, systems thatuse backlights, and methods of making and using backlights. Theinvention is particularly well suited to backlights used in liquidcrystal display (LCD) devices and similar displays, as well as tobacklights that utilize LEDs as a source of illumination.

BACKGROUND

Recent years have seen tremendous growth in the number and variety ofdisplay devices available to the public. Computers (whether desktop,laptop, or notebook), personal digital assistants (PDAs), mobile phones,and thin LCD TVs are but a few examples. Although some of these devicescan use ordinary ambient light to view the display, most include a lightpanel referred to as a backlight to make the display visible.

Many such backlights fall into the categories of “edge-lit” or“direct-lit”. These categories differ in the placement of the lightsources relative to the output area of the backlight, where the outputarea defines the viewable area of the display device. In edge-litbacklights, a light source is disposed along an outer border of thebacklight construction, outside the zone corresponding to the outputarea. The light source typically emits light into a light guide, whichhas length and width dimensions on the order of the output area and fromwhich light is extracted to illuminate the output area. In direct-litbacklights, an array of light sources is disposed directly behind theoutput area, and a diffuser is placed in front of the light sources toprovide a more uniform light output. Some direct-lit backlights alsoincorporate an edge-mounted light, and are thus capable of bothdirect-lit and edge-lit operation.

It is known for direct-lit backlights to use an array of cold cathodefluorescent lamps (CCFLs) as the light sources. It is also known toplace a diffuse white reflector as a back reflector behind the CCFLarray, to increase brightness and presumably also to enhance uniformityacross the output face.

Recently, liquid crystal display television sets (LCD TVs) have beenintroduced that use a direct-lit backlight powered not by CCFLs but byan array of red/green/blue LEDs. An example is the Sony™ Qualia 005 LEDFlat-Screen TV. The 40 inch model uses a direct-lit backlight containingfive horizontal rows of side-emitting Luxeon™ LEDs, each row containing65 such LEDs arranged in a GRBRG repeating pattern, and the rows beingspaced 3.25 inches apart. This backlight is about 42 mm deep, measuredfrom the front of a diffuse white back reflector to the back of a (about2 mm thick) front diffuser, between which is positioned a flattransparent plate having an array of 325 diffuse white reflective spots.Each of these spots, which transmit some light, is aligned with one ofthe LEDs to prevent most of the on-axis light emitted by the LED fromstriking the front diffuser directly. The back reflector is flat, withangled sidewalls.

BRIEF SUMMARY

The present application discloses, inter alia, direct-lit backlightsthat include a back reflector and a transflector that partiallytransmits and partially reflects incident light. The transflector isshaped to form at least one concave structure facing the back reflectorto provide one or more recycling cavities therebetween. In some casesthe backlight may have only one recycling cavity, while in others it canhave a plurality of such cavities. In either case, the single cavity orgroup of cavities are sized and arranged to substantially fill theoutput area of the panel. Further, at least one light source, and insome cases an array of light sources, is disposed behind the output areaof the backlight to inject light into each recycling cavity. In somecases one or more light sources are disposed in a given recyclingcavity; in some cases it or they can be disposed behind the backreflector to inject light into the recycling cavity, for example throughone or more apertures in the back reflector. Advantageously,conventional packaged or unpackaged LEDs can be used as light sources.

The concave nature of the transflector has been found to be particularlyeffective in providing uniform illumination over the area of therecycling cavity, even when using sparsely arranged discrete lightsources such as LEDs. It has also been found to be effective in colormixing light from different colored discrete light sources, such as anarray of individual red/green/blue LEDs.

To minimize the overall thickness of the backlight and the number ofrequired light sources, the transflector's concave shape and itsplacement relative to the back reflector can provide a relativelyshallow and wide recycling cavity. In this regard, the recycling cavitypreferably has a width that is more than two times, and preferably atleast five times or ten times, the recycling cavity's depth. Therecycling cavity can be substantially one-dimensional, forming anextended tunnel-like structure, or two-dimensional, forming a closedcell where the transflector is concave in each of two orthogonalcross-sectional planes. The recycling cavity is also preferably hollowto minimize panel weight.

For enhanced panel efficiency the back reflector is preferably highlyreflective, e.g., at least 90% average reflectivity for visible lightemitted by the light sources, and in exemplary embodiments at least 95,98, or 99% or more. The back reflector can be a predominantly specular,diffuse, or combination specular/diffuse reflector.

The transflector can comprise a variety of partially transmissive andpartially reflective films or bodies, and for enhanced panel efficiencythe transflector desirably has low absorptive losses. Structured surfacefilms such as films having parallel grooves forming extended linearprisms, or films having patterns of pyramidal prisms such as cube cornerelement arrays, are one example. Reflective polarizers, whetherspecularly reflective or diffusely reflective, are another example. Thereflective polarizer may have a coextruded polymeric multilayerconstruction, a cholesteric construction, a wire grid construction, or ablended (continuous/disperse phase) film construction, and thus cantransmit and reflect light specularly or diffusely. A perforatedspecular or diffuse reflective film is another example of a suitabletransflector.

Associated components, systems, and methods are also disclosed.

These and other aspects of the present application will be apparent fromthe detailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the specification, reference is made to the appendeddrawings, where like reference numerals designate like elements, andwherein:

FIG. 1 is a perspective exploded view of a display system that includesa backlight;

FIG. 1 a is a view similar to FIG. 1 but also showing in phantom thelocation of discrete light sources and light recycling cavities disposedbehind the output area of the backlight;

FIG. 2 is a schematic side elevational view of the backlight of FIGS. 1and 1 a;

FIG. 2 a is a view similar to FIG. 2 but showing only selectedcomponents of the backlight, and showing how light rays emitted by thelight sources are recycled within the cavities and emitted through thetransflector;

FIG. 2 b is a fragmentary view showing a light source positioned behindthe back reflector of a recycling cavity;

FIGS. 3 a-g are schematic side elevational views of additionalbacklights;

FIG. 4 is a schematic sectional view of a recycling cavity module foruse in a backlight;

FIG. 5 is a perspective view of another recycling cavity module for usein a backlight;

FIGS. 6 and 7 are front views of different backlights, showing theplacement of multiple light recycling cavities behind the output area ofthe backlights; and

FIGS. 8-11 are views of various packaged LEDs useable as light sourcesin the disclosed backlights.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

One popular application of a backlight is shown schematically in theperspective exploded view of FIG. 1. There, a display system 10 includesa display panel 12, such as a liquid crystal display (LCD) panel, and adirect-lit backlight 14 that provides large area illumination sufficientfor information contained in the display panel to be easily observed.Both display panel 12 and backlight 14 are shown in simplified box-likeform, but the reader will understand that each contains additionaldetail. Backlight 14 emits light over an extended output area 16, andmay also include a frame 15. The output area 16, which is usuallyrectangular but can take on other extended area shapes as desired, maycorrespond to the outer surface of a film used in the backlight, or maysimply correspond to an aperture in the frame 15. In operation, theentire output area 16 is illuminated by light source(s) disposed withinframe 15 but positioned directly behind the output area 16. Whenilluminated, the backlight 14 makes visible for a variety of observers18 a, 18 b an image or graphic provided by display panel 12. In the caseof an LCD panel, the image or graphic is dynamic, produced by an arrayof typically thousands or millions of individual picture elements(pixels), which array substantially fills the lateral dimensions, i.e.the length and width, of the display panel 12. In other embodiments thedisplay panel may be or comprise a film having a static graphic imageprinted thereon. FIG. I also includes a Cartesian x-y-z coordinatesystem for reference purposes.

In some LCD embodiments, the backlight 14 continuously emits white lightand the pixel array is combined with a color filter matrix to formgroups of multicolored pixels (such as yellow/blue (YB) pixels,red/green/blue (RGB) pixels, red/green/blue/white (RGBW) pixels,red/yellow/green/blue (RYGB) pixels, red/yellow/green/cyan/blue (RYGCB)pixels, or the like) so that the displayed image is polychromatic.Alternatively, polychromatic images can be displayed using colorsequential techniques, where, instead of continuously back-illuminatingthe display panel with white light and modulating groups of multicoloredpixels in the display panel to produce color, separate differentlycolored light sources within the backlight itself (selected, forexample, from red, orange, amber, yellow, green, cyan, blue (includingroyal blue), and white in combinations such as those mentioned above)are modulated such that the backlight flashes a spatially uniformcolored light output (such as, for example, red, then green, then blue)in rapid repeating succession. This color-modulated backlight is thencombined with a display module that has only one pixel array (withoutany color filter matrix), the pixel array being modulated synchronouslywith the backlight to produce the whole gamut of achievable colors(given the light sources used in the backlight) over the entire pixelarray, provided the modulation is fast enough to yield temporalcolor-mixing in the visual system of the observer. Examples of colorsequential displays, also known as field sequential displays, aredescribed in U.S. Pat. No. 5,337,068 (Stewart et al.) and U.S. Pat. No.6,762,743 (Yoshihara et al.), hereby incorporated by reference. In somecases, it may be desirable to provide only a monochrome display. Inthose cases the backlight 14 can include filters or specific sourcesthat emit predominantly in one visible wavelength or color.

The display system 10 is shown again in FIG. 1 a, with FIG. 1 aadditionally showing in phantom a first row of discrete light sources 20and a second row of discrete light sources 22 within the direct-litbacklight 14. The light sources 20, 22 may each emit white light, or mayeach emit only one of the RYGCB colors and then either be mixed toprovide a white light output or be matched to provide a monochromeoutput. Also shown in phantom is a boundary 24, which separates a firstrecycling cavity 26, illuminated by sources 20, from a second recyclingcavity 28, illuminated by sources 22. Both recycling cavities, as wellas the sources 20 and 22, are disposed directly behind the output area16.

Collectively, the recycling cavities 26, 28 substantially fill theoutput area 16. Thus, if the output area is depicted in plan view, asfor example when viewed by a distant observer situated along an axisperpendicular to the output area, the summed projected area of theconcave recycling cavities (even though such cavities may not bevisually apparent to the distant observer) is more than half the surfacearea of the output area, preferably at least 75%, 80%, or 90% of theoutput area, more preferably about 100% of the output area. Whether thebacklight has only one concave recycling cavity or a plurality of them,the projected area of the cavity or cavities when viewing the outputarea in plan view accounts for preferably at least 75%, 80%, or 90%, or100%, of the backlight output area.

A schematic side elevational view of selected components of backlight 14is shown in FIG. 2. The two recycling cavities 26, 28 are formed by aback reflector 30 and a transflector 32, the transflector being shapedto form two concave structures as shown, each of which face the backreflector to form the neighboring recycling cavities 26, 28. Discretelight sources 20 are disposed in recycling cavity 26 between the backreflector and the transflector. The placement of the sources 20, 22directly behind the output area 16 is consistent with the backlight 14being of the direct-lit variety. In front of the transflector 32 (fromthe perspective of the observer) are some additional light managementfilms or other components, some or all of which may be optionaldepending on system requirements and characteristics of the lightrecycling cavities and light sources. Thus, in front of the transflectoris a diffuser plate or film 34, and a top film stack comprisingconventional light management films such as a reflective polarizer 36and a prismatic brightness enhancement film 38.

FIG. 2 a is a detail of FIG. 2, showing how light emitted by the lightsources is partially transmitted and partially reflected by thetransflector 32, and how this in combination with the back reflector 30provides light recycling within the cavities 26, 28, as well as lightemission or leakage spread over the lateral dimensions of the cavities.The concave structures of the transflector not only help define theboundaries of the recycling cavities, they also have a tendency toconfine recycled light within those boundaries, and to spread out theangular wedge of emitted light due to the changing geometry of thetransflecting surface. Light confinement within a particular recyclingcavity is a function of design details. For example, light confinementwithin cavities 26, 28 can be increased (and light leakage between thecavities correspondingly decreased) by positioning the transflector 32closer to the back reflector 30, until in the limit the transflectorcontacts the back reflector along a line corresponding to the boundary24 shown in FIG. 1 a. Such positioning is desirable from the standpointof reducing the overall backlight thickness.

Instead of being located inside the cavities, the sources canalternatively be positioned behind the back reflector 30 by translatingthem along the negative z direction, as long as back reflector 30 isprovided with suitable apertures, such as corresponding holes, slots,windows, or other light-transmitting areas, so that light from thesources can still be directly injected into the cavities. This is shownin the fragmentary view of FIG. 2 b.

As discussed in further detail below, a given light source can be (1) anactive component such as an LED die or fluorescent lamp that convertselectricity to light or a phosphor that converts excitation light toemitted light, or (2) a passive component such as a lens, waveguide(such as a fiber), or other optical element that transports and/orshapes the light emitted by an active component, or (3) a combination ofone or more active and passive components. For example, light sources20, 22 in FIGS. 2 and 2 a may be packaged side-emitting LEDs in which anLED die is disposed behind the back reflector 30 proximate a circuitboard or heat sink, but a shaped encapsulant or lens portion of thepackaged LED is disposed in the recycling cavity by extending through aslot or aperture in the back reflector. More discussion of light sourcesis provided below.

In the embodiment of FIGS. 1-2 a, the recycling cavities aresubstantially one-dimensional, extending across the output area 16 inadjacent strips that run parallel to the x-axis. The transflector 32 isshaped to form the depicted concave structures in the y-z sectionalplane, but in an orthogonal x-z sectional plane the transflector issubstantially straight and flat. Stated differently, the transflectorexhibits simple curvature. In other embodiments, the transflector canexhibit compound curvature, wherein it is shaped to form concavestructures in both the y-z and x-z sectional planes.

Back reflector 30 is preferably highly reflective for enhanced panelefficiency. For example, the back reflector may have an averagereflectivity for visible light emitted by the light sources of at least90%, 95%, 98%, or 99% or more. The back reflector can be a predominantlyspecular, diffuse, or combination specular/diffuse reflector, whetherspatially uniform or patterned. In some cases the back reflector can bemade from a stiff metal substrate with a high reflectivity coating, or ahigh reflectivity film laminated to a supporting substrate. Suitablehigh reflectivity materials include, without limitation: Vikuiti™Enhanced Specular Reflector (ESR) multilayer polymeric film availablefrom 3M Company; a film made by laminating a barium sulfate-loadedpolyethylene terephthalate film (2 mils thick) to Vikuiti™ ESR filmusing a 0.4 mil thick isooctylacrylate acrylic acid pressure sensitiveadhesive, the resulting laminate film referred to herein as “EDR II”film; E-60 series Lumirror™ polyester film available from TorayIndustries, Inc.; porous polytetrafluoroethylene (PTFE) films, such asthose available from W. L. Gore & Associates, Inc.; Spectralon™reflectance material available from Labsphere, Inc.; Miro™ anodizedaluminum films (including Miro™ 2 film) available from AlanodAluminum-Veredlung GmbH & Co.; MCPET high reflectivity foamed sheetingfrom Furukawa Electric Co., Ltd.; and White Refstar™ films and MT filmsavailable from Mitsui Chemicals, Inc. The back reflector may besubstantially flat and smooth, or it may have a structured surfaceassociated with it to enhance light scattering or mixing. Such astructured surface can be imparted (a) on the reflective surface of theback reflector, or (b) on a transparent coating applied to thereflective surface. In the former case, a highly reflecting film may belaminated to a substrate in which a structured surface was previouslyformed, or a highly reflecting film may be laminated to a flat substrate(such as a thin metal sheet, as with Vikuiti™ Durable Enhanced SpecularReflector-Metal (DESR-M) reflector available from 3M Company) followedby forming the structured surface such as with a stamping operation. Inthe latter case, a transparent film having a structured surface can belaminated to a flat reflective surface, or a transparent film can beapplied to the reflector and then afterwards a structured surfaceimparted to the top of the transparent film.

The back reflector can be a continuous unitary (and unbroken) layer onwhich the light source(s) are mounted, or it can be constructeddiscontinuously in separate pieces, or discontinuously insofar as itincludes isolated apertures, through which light sources can protrude,in an otherwise continuous layer. For example, strips of reflectivematerial can be applied to a substrate on which rows of LEDs aremounted, each strip having a width sufficient to extend from one row ofLEDs to another, and having a length dimension sufficient to spanbetween opposed borders of the backlight's output area.

Sides and ends of the recycling cavities located along the outerboundary of the output area 16 are preferably lined or otherwiseprovided with high reflectivity vertical walls to reduce light loss andimprove recycling efficiency. Two opposed side walls can be seen at theleft and right extremities of FIG. 2; two opposed end walls, one ofwhich is visible in FIG. 2, are orthogonal to the side walls in thedepicted embodiment. The same reflective material used for the backreflector can be used to form these walls, or a different reflectivematerial can be used. In exemplary embodiments the side walls arediffusely reflective.

The transflector is or comprises a structure such as a film thatpartially transmits and partially reflects incident light, where thepartial transmission is high enough to permit efficient extraction oflight through the transflector, but the partial reflection is also highenough to support light recycling when combined with a back reflector. Avariety of different films can be used, as discussed below, with theoptimal geometry and characteristics in general being different foreach, and being a function of the light sources used and the backreflector, so as to achieve optimal luminance, luminance uniformity(source hiding), and color mixing. (In some cases the backlight designermay be presented with a particular recycling cavity design, such as acavity module discussed below, and may then select suitable sources foruse with the given cavity.) Some suitable films will now be explainedfurther, but the discussion is not intended to be limiting, and any ofthe described films can be used singly or in combination with others toproduce the desired transmission and reflection properties. Forcombinations of films, the films may or may not be attached to eachother. If they are attached, any known attachment mechanisms may beused, and they may be attached over their entire major surfaces or onlyat discrete points or lines. If adhesives are used, the adhesive can betransparent, diffusive, and/or birefringent.

Some of the films suitable for use as a transflector fall intocategories referred to herein as semi-reflective films and lightdeflecting films.

Generally, semi-reflective films refer to films and the like thatreflect on the order of 30 to 90% of normally incident visible light,and have low enough absorption that they transmit a substantial portion,preferably substantially all, of the remaining (non-reflected) light.Reflection and transmission can be specular, diffuse, or a combinationthereof, whether spatially uniform or patterned. Diffuse reflection canbe produced using surface diffusers (including holographic diffusers),bulk diffusers, or both. The appropriate level of reflectivity candepend on a variety of factors including the number of light sources andtheir placement on or at the back reflector, the intensity and theemission profile (angular distribution of intensity) of the source(s),the depth of the recycling cavity, the desired degree of brightness andcolor uniformity in the output of the backlight, and the presence orabsence of other components such as a diffuser plate or a top film stackin the backlight. Higher reflectivity films used for the transflectortend to enhance brightness uniformity and color uniformity of thebacklight at the expense of efficiency. The decrease in efficiencyoccurs because the average number of reflections within the recyclingcavity increases, and each reflection is associated with at least someloss. As mentioned previously, it is desirable to minimize visible lightabsorption not only of the transflector, but also of the back reflectorand any reflective side walls.

One example of a semi-reflective film suitable as a transflector is athin metallized mirror, where the metal coating is thin enough totransmit some visible light. The thin metal coating can be applied to afilm or to a plate substrate.

Another example of a semi-reflective film is referred to in the art as acontrolled transmission mirror film (CTMF). Such a film is made byapplying diffusely reflective coatings or layers to both sides of amultilayer interference mirror stack, such as the ESR mirror filmmentioned above. Another example of a semi-reflective film is amultilayer polymer mirror film that has been flame embossed to disruptthe multilayer interference stack in some places by brief exposure to aflame.

Reflective polarizers are still other examples of a semi-reflectivefilms. Such polarizers, which include cholesteric polarizers, multilayerpolymeric polarizers made by coextrusion and stretching techniques, wiregrid polarizers, and diffuse blend polarizers having acontinuous/disperse phase construction, transmit nominally half of thelight from an unpolarized source (corresponding to a first polarizationstate) and reflect nominally the other half (corresponding to anorthogonal second polarization state). Examples include any of the dualbrightness enhancement film (DBEF) products and any of the diffuselyreflective polarizing film (DRPF) products available from 3M Companyunder the Vikuiti brand. See also, for example, the reflective filmsdisclosed in U.S. Pat. No. 5,882,774 (Jonza et al.) and U.S. Pat. No.6,111,696 (Allen et al.), and in U.S. Patent Publication 2002/0190406(Merrill et al.). If one reflective polarizing film is inadequate, twoor more such films can be combined and then shaped to form the concavestructure(s).

Non-polarizing diffuse reflectors are still more examples ofsemi-reflective films. Such reflectors can be made by dispersingspecularly reflective particles or flakes in a low absorption,transparent polymer matrix, forming a film or other body. The reflectiveparticles or flakes can be distributed through the thickness of a thickfilm, or can be disposed on a surface of a substrate as a thin curablecoating. Numerous other diffuse reflector constructions and methods ofmaking are also known. Diffuse coatings can be applied to reflectors orother bodies by ink-jet printing, screen printing, and other knowntechniques. Diffuse adhesives can also be used, where the diffusion isproduced by refractive index mismatched particles, or air voids. Diffusereflectors used for the transflector preferably have low absorption andaverage transmission values over visible wavelengths from 20% to 80%.

Semi-reflective films also include reflective films that have beenprovided with a pattern of fine holes or apertures to increasetransmission and reduce reflection. This can be done by simplyperforating a reflective film in a desired pattern. Virtually any of thereflective films discussed herein can be used as a starting material,and then converted or processed to provide the perforations or otherapertures. U.S. Patent Application Publications US 2004/0070100 (Strobelet al.) and US 2005/0073070 (Getschel et al.) teach suitable techniquesfor flame-perforating films. The pattern of holes or apertures can beuniform or non-uniform, and in the latter case both the position and thehole size can be random or pseudo-random. In one example, a sheet ofVikuiti™ ESR film is perforated with uniformly spaced round holes, theholes positioned in a hexagonal array with a hole-to-hole spacing equalto a multiple of the hole diameter. From a manufacturing standpoint itis desirable not to have to align the transflector with the lightsource(s), and thus it can be advantageous to use a uniform hole patternand during construction of the backlight make no attempt to register thelight source(s) in any particular way relative to the pattern. Incertain backlight configurations, however, it may be acceptable to use anon-uniform pattern of holes and then position the transflector suchthat certain features of the non-uniform pattern, such as areas havingfewer holes or smaller holes than other areas, are aligned with thelight sources. Further, in some embodiments, the transflector caninclude a reflective film such as ESR that is formed into individualstrips or discrete segments. See, e.g., US 2004/0233665 (West et al.).

Generally, light deflecting films suitable as a transflector in thedisclosed backlights refer to films and the like having minutestructures arranged to form a structured surface or the like thatreflects and transmits light as a function of the direction of incidenceof the light. One or both sides of the film can have such a structuredsurface. Useful structures include linear prisms, pyramidal prisms,cones, and ellipsoids, which structures may be in the form ofprojections extending out from a surface or pits extending into thesurface. The size, shape, geometry, orientation, and spacing of thestructures can all be selected to optimize the performance of thetransflector, recycling cavity, and backlight, and the individualstructures can be symmetric or asymmetric. The structured surface can beuniform or non-uniform, and in the latter case both the position andsize of the structures can be random or pseudo-random. Disruptingregular features by periodic or pseudo-random variation of size, shape,geometry, orientation, and/or spacing may be used to adjust the colorand brightness uniformity of the backlight. In some cases it may bebeneficial to have a distribution of small and large structures andposition the light deflecting film such that the smaller structures arealigned generally over the light sources and the larger structures arepositioned elsewhere.

Examples of suitable light deflecting films include commercialone-dimensional (linear) prismatic polymeric films such as Vikuiti™brightness enhancement films (BEF), Vikuiti™ transmissive right anglefilms (TRAF), Vikuiti™ image directing films (IDF), and Vikuiti™ opticallighting films (OLF), all available from 3M Company, as well asconventional lenticular linear lens arrays. In the case of theseone-dimensional prismatic films, the prismatic structured surfacepreferably faces downward toward the light sources (negative z directionin FIG. 2), and if the transflector is simply curved such that it formsa concave structure in a first sectional plane (such as the y-z plane)but not in an orthogonal second sectional plane (such as the x-z plane),the one-dimensional prismatic film is preferably oriented such that thelinear prisms of its structured surface extend perpendicular to thefirst plane (e.g., the y-z plane) and parallel to the second plane (e.g.the x-z plane). Further examples of light deflecting films, where thestructured surface has a two-dimensional character, include: cube comersurface configurations such as those disclosed in U.S. Pat. No.4,588,258 (Hoopman), U.S. Pat. No. 4,775,219 (Appeldorn et al.), U.S.Pat. No. 5,138,488 (Szczech), U.S. Pat. No. 5,122,902 (Benson), U.S.Pat. No. 5,450,285 (Smith et al.), and U.S. Pat. No. 5,840,405 (Shustaet al.); unsealed cube comer sheeting such as 3M™ Scotchlite™ ReflectiveMaterial 6260 High Gloss Film and 3M™ Scotchlite Reflective Material6560 High Gloss Sparkle Film, available from 3M Company; inverted prismsurface configurations such as described in U.S. Pat. No. 6,287,670(Benson et al.) and U.S. Pat. No. 6,280,822 (Smith et al.); structuredsurface films disclosed in U.S. Pat. No. 6,752,505 (Parker et al.) andpatent publication US 2005/0024754 (Epstein et al.); and beadedretroreflective sheeting.

The light deflecting films can be used alone or in combination withother suitable transflectors. If used in combination with a differenttype of transflector, the light deflecting film can be positioned to beon the interior of the recycling cavity (closest to the light sources),and the other film, which may be a semi-reflecting film (for example, adiffusing film) or another light deflecting film, can be positioned onthe exterior of the recycling cavity. If two or more linear prismaticlight deflecting films are combined, they can be aligned, misaligned, or“crossed” such that the prism direction of one film is perpendicular tothe prism direction of the other film.

Returning now to FIGS. 2 and 2 a, the discrete light sources 20, 22 areshown schematically. In most cases, these sources are compact lightemitting diodes (LEDs). In this regard, “LED” refers to a diode thatemits light, whether visible, ultraviolet, or infrared. It includesincoherent encased or encapsulated semiconductor devices marketed as“LEDs”, whether of the conventional or super radiant variety. If the LEDemits non-visible light such as ultraviolet light, and in some caseswhere it emits visible light, it is packaged to include a phosphor (orit may illuminate a remotely disposed phosphor) to convert shortwavelength light to longer wavelength visible light, in some casesyielding a device that emits white light. An “LED die” is an LED in itsmost basic form, i.e., in the form of an individual component or chipmade by semiconductor processing procedures. The component or chip caninclude electrical contacts suitable for application of power toenergize the device. The individual layers and other functional elementsof the component or chip are typically formed on the wafer scale, andthe finished wafer can then be diced into individual piece parts toyield a multiplicity of LED dies. More discussion of packaged LEDs,including forward-emitting and side-emitting LEDs, is provided below.

If desired, other visible light emitters such as linear cold cathodefluorescent lamps (CCFLs) or hot cathode fluorescent lamps (HCFLs) canbe used instead of or in addition to discrete LED sources asillumination sources for the disclosed backlights. For example, in someapplications it may be desirable to replace the row of discrete lightsources 20 seen in FIG. 1 a with a different light source such as a longcylindrical CCFL, or with a linear surface emitting light guide emittinglight along its length and coupled to a remote active component (such asan LED die or halogen bulb), and to do likewise with the row of discretesources 22. Examples of such linear surface emitting light guides aredisclosed in U.S. Pat. No. 5,845,038 (Lundin et al.) and U.S. Pat. No.6,367,941 (Lea et al.). Fiber-coupled laser diode and othersemiconductor emitters are also known, and in those cases the output endof the fiber optic waveguide can be considered to be a light source withrespect to its placement in the disclosed recycling cavities orotherwise behind the output area of the backlight. The same is also trueof other passive optical components having small emitting areas such aslenses, deflectors, narrow light guides, and the like that give offlight received from an active component such as a bulb or LED die. Oneexample of such a passive component is a molded encapsulant or lens of aside-emitting packaged LED.

Turning now to FIGS. 3 a-g, we see there a small sample of the widevariety of different geometrical configurations with which one canconstruct suitable direct-lit backlights. The figures are allrepresented as elevational views directed along the x-axis, which isperpendicular to the plane of the figures. However, the figures can alsobe interpreted to represent elevational views directed along theorthogonal y-axis, thus generally depicting both embodiments in whichthe transflector has simple curvature in the y-z plane, as well as thosein which the transflector has compound curvature in both the y-z and x-zplanes. In this regard, “curvature” should be understood broadly, and isnot limited to circular geometric arcs or even to curved shapes.

FIG. 3 a shows a direct-lit backlight 40 having only one recyclingcavity 42 formed by a single concave structure in a transflector 44, incombination with the back reflector 30. The recycling cavity has a depthd as shown and a length and width substantially equal to the length andwidth of the output area 16, which now is located at the front surfaceof diffuser plate 34. Three light sources 45 a, 45 b, 45 c are disposedin the cavity 42, and each may represent a single light source or a rowof light sources extending parallel to the x-axis. Depending on thebrightness and spatial and angular distribution of emitted light, anyone or two of sources 45 a, 45 b, 45 c may be omitted and still provideacceptable brightness and brightness uniformity at the output area 16.

FIG. 3 b shows a direct-lit backlight 50 having two recycling cavities52, 54 formed by two concave structures in a transflector 56, incombination with the back reflector 30. The transflector 56 is shown intwo parts, 56 a and 56 b, corresponding to the two concave structures.These parts may or may not be connected by a portion of the transflectorin the intermediate region 53. As seen by the presence of theintermediate region 53, and the region to the left of cavity 52 and theregion to the right of cavity 54, the cavities 52, 54 do notcollectively fill the entire output area 16. Nevertheless, the cavities52, 54 are still sized to substantially fill the output area, preferablyaccounting for 75%, 80%, or 90% or more of the plan view area of theoutput area 16. Regions disposed behind the output area 16 that lackconcave recycling cavities, such as intermediate region 53, collectivelyamount to a small percentage (less than 25%, 20%, or 10%, and preferablyabout 0%) of the plan view area of the output area. These regions mayhave little or no detrimental effect on the brightness uniformity acrossthe output area 16 because of the proximity of the recycling cavit(ies),the angular distribution of light emitted by the recycling cavit(ies),and the position of the output area above the recycling cavit(ies)(e.g., the placement of diffuser plate 34 ). To the extent the regionsare present, in exemplary embodiments they are distributedpreferentially near or along the periphery of the output area 16 andaway from the central portion of the output area. Each recycling cavityhas a depth d as shown. Two light sources 55 a, 55 b are disposed incavity 52, and two light sources 55 c, 55 d are disposed in cavity 54,but one light source per cavity may also be sufficient. The illustratedlight sources may represent a single light source or a row of lightsources extending parallel to the x-axis.

FIG. 3 c shows a direct-lit backlight 60 having three recycling cavities62, 64, 66 formed by three concave structures in a transflector 68, incombination with the back reflector 30. The transflector 68 is shown inthree parts, 68 a, 68 b, and 68 c, corresponding to the three concavestructures. These parts may or may not be connected by portions of thetransflector in the intermediate regions 63 a, 63 b, the area of whichis preferably minimized. Note that the concave structures are eachcomposed of distinct left and right halves, each half having a convexshape relative to the back reflector 30 but two halves together forminga structure that is concave relative to the back reflector. Eachrecycling cavity has a depth d as shown. Light sources 65 a, 65 b, 65 care disposed in their respective cavities 62, 64, 66. The illustratedlight sources may represent a single light source or a row of lightsources extending parallel to the x-axis.

FIG. 3 d shows a direct-lit backlight 70 having three recycling cavities72, 74, 76 formed by three concave structures in a transflector 78, incombination with the back reflector 30. The transflector 78 is shown inthree parts, 78 a, 78 b, and 78 c, corresponding to the three concavestructures. These parts may or may not be connected by portions of thetransflector in the intermediate regions 73 a, 73 b, the area of whichis preferably minimized. Note that the concave structures are eachcomposed of distinct left, right, and top portions, the top portionsbeing connected to the other two portions by attachment mechanisms 77.These attachment mechanisms 77 can be of conventional design, forexample, a molded plastic frame. Attachment mechanisms 77 can also betransparent or opaque, and can be larger or smaller than the relativesize shown in the figure. Each recycling cavity has a depth d as shown.Light sources 75 a, 75 b, 75 c are disposed in their respective cavities72, 74, 76. The illustrated light sources may represent a single lightsource or a row of light sources extending parallel to the x-axis.

FIG. 3 e shows another direct-lit backlight 80, having four recyclingcavities 82, 84, 86, 88 formed by four concave structures in atransflector 89, in combination with the back reflector 30. Thetransflector 89, which rests atop or attaches to a transparent support81, is shown in four parts, 89 a-d, corresponding to the four concavestructures. These parts may or may not be part of a continuoustransflective film. Each recycling cavity has a depth d as shown. Incertain embodiments, the transparent support 81 may have an engineeredsurface structure facing the light sources 85 a-d. Light sources 85 a-dare disposed in their respective cavities 82, 84, 86, 88. Theillustrated light sources may represent a single light source or a rowof light sources extending parallel to the x-axis. In an alternativeembodiment transparent support 81 may be replaced with a back reflectorhaving apertures therein to permit the light sources to illuminate thecavities, and back reflector 30 may be replaced with a substrate such asa circuit board on which to mount the light sources. In that case thedepth of the recycling cavities decreases to d′ as shown.

FIG. 3 f shows another direct-lit backlight 90, having two outerrecycling cavities 92 a, 94 a and two inner recycling cavities 92 b, 94b, the outer cavities being formed by two concave structures in a firsttransflector 96 in combination with the back reflector 30, and the innercavities being formed by two concave structures in a second transflector98 in combination with smaller portions of the same back reflector 30.Transflector 96 is shown in two parts 96 a, 96 b, which may or may notbe connected by a portion of the transflector 96 in an intermediateregion 93. Transflector 98 is also shown in two parts 98 a, 98 b, whichlikewise may or may not be connected by a portion of the transflector 98in the intermediate region 93. The outer and inner transflectors can usethe same type of transflective material, e.g., a particularsemi-reflective film or a particular light deflecting film, or they mayuse different materials, such as a prismatic light deflecting film forone and a perforated reflector for the other. The outer recyclingcavities have a depth d1 as shown. The inner recycling cavities have asmaller depth d2. Light source 95 a is disposed in both outer cavity 92a and inner cavity 92 b; light source 95 b is disposed in both outercavity 94 a and inner cavity 94 b. The illustrated light sources mayrepresent a single light source or a row of light sources extendingparallel to the x-axis.

FIG. 3 g shows still another direct-lit backlight 100, having two outerrecycling cavities 102 a, 104 a and two inner recycling cavities 102 b,104 b, the outer cavities being formed by two concave structures in afirst transflector 106 in combination with the back reflector 30, andthe inner cavities being formed by two concave structures in a secondtransflector 108. Transflector 106 is shown in two parts 106 a, 106 b;transflector 108 is also shown in two parts 108 a, 108 b. FIG. 3 gexemplifies a construction technique in which a concave (and curved)structure is formed by flexing an otherwise flat stiff film and holdingit in compression between fixed posts. Thus, transflectors 106 a and 108a are held in compression between fixed posts 107 a, 107 b, resulting innested cavities of equal width but different depth. Transflectors 106 band 108 b are held in compression between fixed posts 107 b, 107 c, alsoresulting in nested cavities of equal width but different depth. Theouter and inner transflectors can use the same type of transflectivematerial, e.g., a particular semi-reflective film or a particular lightdeflecting film, or they may use different materials, such as aprismatic light deflecting film for one and a perforated reflector forthe other. The outer recycling cavities have a depth d1, the innerrecycling cavities have a smaller depth d2. Light source 105 a isdisposed in both outer cavity 102 a and inner cavity 102 b; light source105 b is disposed in both outer cavity 104 a and inner cavity 104 b. Theillustrated light sources may represent a single light source or a rowof light sources extending parallel to the x-axis.

FIG. 4 shows a recycling cavity module 110. Module 110 has threerecycling cavities 112, 114, 116, formed by three concave structures ina transflector 118, in combination with a back reflector 119. Thetransflector 118, which is coated or laminated to a stiff transparentsupport 117, is shown in three parts, 118 a-c, corresponding to thethree concave structures. The transflector 118 is shown to becontinuous, but it can also be discontinuous, for example, omitted fromintermediate regions 115 a, 115 b, or from selected portions of theconcave structures. An adhesive or other bonding mechanism can be usedin the regions 115 a, 115 b and at the left and right extremities of themodule 110 to secure the transflector 118 to the back reflector 119.Back reflector 119 is coated or laminated to a stiff support 113. Thecombination back reflector 119/support 113 has apertures 111 a-ctherein, which may be physical holes or slots, or alternatively portionsof light-transmissive film or other material. The apertures are sized toreceive suitable light sources so that the module 110 can be simplydropped in place over an array of light sources for fast and easybacklight construction. A single module 110 can be used to construct abacklight having three light sources or three rows of light sources;multiple modules can also be used on larger backlights, e.g., twomodules 110 can be used side-by-side to construct a backlight having sixlight sources or six rows or light sources. Each recycling cavity has adepth d as shown.

FIG. 5 shows a recycling cavity module 120 similar to the module of FIG.4, except that module 120 has only one recycling cavity 122. Cavity 122is formed by one concave structure in a transflector 124 in combinationwith a back reflector 126. The transflector 124 can be coated orlaminated to a stiff transparent support 125. The transflector 124 isshown to be continuous, but it can also be discontinuous, for example,omitted from the opposed edges of the module extending parallel to thex-axis, or from selected portions of the concave structure. An adhesiveor other bonding mechanism can be used along the edges of the module 110to secure the transflector 124 to the back reflector 126. Back reflector126 is coated or laminated to a stiff support 127. The combination backreflector 126/support 127 has apertures 128 a-d therein, which may bephysical holes, or alternatively portions of light-transmissive film orother material. The apertures are sized to receive suitable lightsources so that the module 120 can be simply dropped in place over anarray (in this case, a row) of light sources for fast and easy backlightconstruction. A single module 120 can be used to construct a backlighthaving one row of four light sources; multiple modules 120 can also beused on larger backlights, e.g., placed end-to-end and/or side-by-sideto construct backlights having n rows of four light sources, where n isany positive integer, or n rows of 4 m light sources, where m is alsoany positive integer. The recycling cavity 122 has a depth d, and alength L and a width W as shown.

Modules of differing design, and more generally cavities of differingdesign, can be mixed and matched as desired in a given backlight.Multiple cavities in a given backlight need not have the same shape inplan view, they need not have the same length, width, or depth, theyneed not be oriented in the same way, and they need not use the sametransflector material or back reflector material. Also, whether or notthe cavities have the same geometrical and optical properties, they neednot have the same number of light sources or the same type of lightsources. The sizes, positions, orientations, and other features of thecavities (and of cavity modules) can be selected to yield the desiredcharacteristics of the backlight.

The recycling cavities need not have a rectangular outline in plan view.For example, recycling cavity 122 of FIG. 5 can be modified by makingthe cavity wider in the center proximate apertures 128 b, 128 c andnarrower at the ends proximate apertures 128 a, 128 d to produce abarrel-shaped plan view outline, or the cavity can be made narrower inthe center and wider at the ends to produce an hour-glass-shaped planview outline.

FIGS. 6 and 7 are front or plan views of different direct-litbacklights, showing the placement of multiple light recycling cavitiesbehind the output area of the panels. The output areas of each backlighthave a 16:9 aspect ratio, which is currently popular in LCD TVs. In FIG.6, a backlight output area 130 is substantially filled by an array ofsix recycling cavities 132 a-f. Each recycling cavity is formed by atransflector shaped to form a concave structure facing a back reflector.The transflector is shaped to define one concave structure in the x-zplane and another concave structure in the orthogonal y-z plane, theformer defining a width W and the latter defining a length L of eachrecycling cavity. Light sources 134 are disposed in the recyclingcavities or behind the back reflector.

FIG. 6 can also be construed to show embodiments in which thetransflector has simple curvature to define one or more lineartunnel-like structures, but where vertical partitions are disposedbetween the transflector and the back reflector to segment a recyclingcavity into separate zones or sub-cavities. For example, thetransflector may form a single concave structure in the x-z planebetween the top and bottom edges of output area 130, forming a recyclingcavity of width 2W and length 3L (where W and L are as depicted in FIG.6), except that vertical partitions, preferably made of a highlyreflective material, whether specular or diffuse, are arranged betweenthe transflector and the back reflector as shown by the broken lines todefine distinct zones or cavities 132 a-f. As another example, thetransflector may form two adjacent concave structures in the x-z planeto form two recycling cavities, each having a width W and a length 3L(where W and L are as depicted in FIG. 6), except that verticalpartitions are disposed between the transflector and the back reflectorto segment the first cavity into three cavities 132 a-c, and to segmentthe second cavity into three cavities 132 d-f. In still another example,the transflector may form three adjacent concave structures in the y-zplane to form three recycling cavities, each having a width L and alength 2W (where W and L are as depicted in FIG. 6), except that avertical partition is disposed between the transflector and the backreflector to segment each cavity into two cavities.

In FIG. 7, a panel output area 140 is substantially filled by an arrayof ten hexagonal recycling cavities 142 a-j. Each recycling cavity isformed by a transflector shaped to form a concave structure facing aback reflector. The transflector is shaped to define one concavestructure in the x-z plane and another concave structure in theorthogonal y-z plane, the former defining a width W and the latterdefining a length L of each recycling cavity. Light sources 144 aredisposed in the recycling cavities or behind the back reflector.

Besides rectangular and hexagonal shapes, other plan-view shapes can beused for the recycling cavities, whether they are simply curved orcomplex curved. Polygons (triangles, trapezoids, pentagons, etc.),circles, ellipses, and any other desired shapes are contemplated. Thegeometry can be tailored to achieve high efficiency and brightness andcolor uniformity in the backlight.

As mentioned previously, the recycling cavities formed by the concavetransflector and back reflector are desirably relatively shallow in thez-direction (i.e., small depth d) and relatively wide in a transversedirection. Depth d of a particular cavity refers to the maximumseparation in that cavity between the back reflector and thetransflector along an axis perpendicular to the output area, i.e., alongthe z-direction. Width (W) of a cavity refers to a lateral dimension ofthe cavity measured as follows: beginning with the shape of the cavityin plan view (e.g., FIGS. 6 and 7), the width of the cavity is the minordimension of the smallest rectangle that can circumscribe the plan viewcavity shape. The disclosed recycling cavities desirably have widths Wgreater than 2 d, preferably at least 5 d or 10 d or more. Length (L) ofa cavity refers to the major dimension of the smallest rectangle thatcan circumscribe the plan view cavity shape. In special cases thesmallest rectangle may be a square, in which case L=W.

Backlights utilizing more than one of the disclosed recycling cavities,and particularly those having zones or arrays of distinct cavities, eachof which is illuminated by its own light source(s) which are separatelycontrolled or addressable relative to light source(s) in neighboringcavities, can be used with suitable drive electronics to support dynamiccontrast display techniques and color sequential display techniques, inwhich the brightness and/or color distribution across the output area ofthe backlight is intentionally non-uniform. Thus, different zones of theoutput area can be controlled to be brighter or darker than other zones,or the zones can emit in different colors, simply by appropriate controlof the different light sources in the different recycling cavities.

The disclosed concave recycling cavities can be fabricated forbacklights using a wide variety of assembly methods and techniques.

In one method, a single piece of transflective film spans the entirewidth of a backlight enclosure, where the edges of the film are wedgedbetween or are physically affixed to sidewalls of the enclosure to forma concave tunnel-like structure. This method is particularly suited torelatively small displays.

In the case of thin and wide backlight units, it can be advantageous touse multiple concave tunnel-like structures. Scoring a transflectivefilm, i.e., cutting through a portion of the film's thickness along oneor more lines, has been found to be a convenient technique for definingthe boundaries of the concave structures. Another useful technique iscreasing the transflective film by folding it along one or more lines.Scoring and creasing can facilitate the assembly of multiple concavestructures from a single film by providing defined positions at whichthe film is predisposed to fold. Scoring can be accomplished by anyknown scoring technique, including laser methods, thermal methods suchas hot wire or hot knife, and known kiss-cutting techniques.

When using multiple tunnel-like structures formed from a single film,physical attachment of the film to the backplane, sidewalls, or both thebackplane and sidewalls of the backlight enclosure can provide the filmwith a stable and robust structure. Examples of methods for physicalattachment of a concave film to a backlight include, but are not limitedto: pinning scored sections of the film to the backplane via rivets,screws, staples, thermal or ultrasonic spot welds, plastic pins thatsnap into the backplane (which may also be used to support the diffuserplate, as in the Sony™ Qualia LED LCD TV), pins that snap into thesidewalls of the backlight and pin the scored areas of the film to thebackplane, adhesive strips on the backplane, and the like.

Edges of the concave film can be attached to positions or slots moldedinto the sidewall reflectors of the backlight enclosure that help definethe shape of the concave structure. Alternatively, the film can beprepared to be rigid enough so that the concave structure can snap intopredefined slots in the sidewalls or reflective backplane. The stiffnessor rigidity of a transflector can be enhanced, in general, bycorrugating at least a portion of the transflector. A transflector thatlacks sufficient stiffness by itself can also be combined with (e.g.,placed atop) a transparent support having a suitable surface shape.

Another approach to secure a scored film into a backlight enclosureinvolves the use of supporting members, which can be molded into thesidewall structure of the enclosure or snap into the sidewalls of theenclosure. This method can utilize transparent polymer rods that snapinto predefined positions in the sidewalls of the enclosure, spanning alength or width dimension of the backlight, thus providing guidesthrough which a transflector can be woven or threaded, with the filmbeing secured at its edges using techniques described above.Alternatively, the rods can be replaced with fine gauge wire. Thisapproach can be particularly useful for making asymmetric concavestructures, where the film would normally resist taking on an asymmetricshape.

Another approach to form the concave structures in the transflector isto place plastic pins at predetermined locations on the back side of afront diffuser plate corresponding to places at which the transflectoris intended to contact or nearly contact the back reflector. Duringbacklight assembly, the pins can contact a flexible transflective filmto form the transflector, which can be attached at its edges to thebacklight enclosure, into one or more concave shapes defined by thepositions of the pins.

Other methods for forming the concave structures involve fabricatingmodules that are separate from the backlight. A module similar to thatshown in FIG. 4, can be made wherein a transflector is attached atpoints to unconnected reflective backplane pieces which, when broughttogether, leaves an aperture for the light source(s). Separate pieces ofthe back reflector, such as shown in FIG. 4, can also be connected byz-fold hinges such that when the hinges are extended, the transflectorcan lie flat for shipping purposes, and when the hinges are folded, theconcave structure is formed, leaving apertures for the light source(s).Another type of module is shown in FIG. 5, where the back reflector canbe a reflective film such as ESR film from 3M Company with a definedlength and having apertures through which light sources can protrude. Atransflective film wider than the back reflector film is introduced, andthe two films are bonded or welded along two opposed edges, forming aconcave, tunnel-like structure. The module can be attached to thebacklight enclosure by conventional means such as an adhesive applied tothe back surface of the back reflector.

Other methods for forming concave structures are described in commonlyassigned U.S. Application entitled “Methods of forming Direct-litBacklights having Light Recycling Cavity with Concave Transflector”(Attorney Docket No. 61199US002), filed on even date herewith.

FIGS. 8-11 show views of some light sources that are useable in thedisclosed backlights, but they are not intended to be limiting. Theillustrated light sources comprise packaged LEDs. The light sources ofFIGS. 8, 9, and 11 show side-emitting LED packages, where light from anLED die is reflected and/or refracted by an integral encapsulant or lenselement to provide peak light emission in a generally lateral directionrather than forward along a symmetry axis of the source. The lightsource of FIG. 10 can be forward emitting or side-emitting, depending onwhether an optional deflector is included.

In FIG. 8, a light source 150 includes an LED die 151 carried by a frame152 and electrically connected to leads 153. Leads 153 are used toelectrically and physically connect the light source 150 to a circuitboard or the like. A lens 154 is attached to frame 152. The lens 154 isdesigned such that light emitted into an upper section of the lens istotally internally reflected on an upper surface 155 such that it isincident on a bottom surface 156 of the upper section and refracted outof the device. Light emitted into a lower section 157 of the lens isalso refracted out of the device. Light source 150 can also include anoptional diverter 158, such as a disk of reflective material, mountedabove the lens 154 or attached to the upper surface 155. See also U.S.Patent Application Publication US 2004/0233665 (West et al.).

In FIG. 9, a light source 160 includes an LED die (not shown) mounted ona lead frame 161. A transparent encapsulant 162 encapsulates the LEDdie, lead frame 161, and a portion of the electrical leads. Theencapsulant 162 exhibits reflection symmetry about a plane containing anLED die surface normal. The encapsulant has a depression 163 defined bycurved surfaces 164. Depression 163 is essentially linear, centered onthe plane of symmetry, and a reflective coating 165 is disposed on atleast a portion of surface 164. Light emanating from the LED diereflects off reflective coating 165 to form reflected rays which are inturn refracted by a refracting surface 166 of the encapsulant, formingrefracted rays 167. See also U.S. Pat. 6,674,096 (Sommers).

In FIG. 10, a light source 170 includes an LED die 171 disposed in arecessed reflector area 172 of a lead frame 173. Electrical power issupplied to the source by the lead frame 173 and another lead frame 174,by virtue of wire bond connections from the lead frames to the LED die171. The LED die has a layer of fluorescent material 175 over it, andthe entire assembly is embedded in a transparent encapsulation epoxyresin 176 having a lensed front surface. When energized, the top surfaceof the LED die 171 produces blue light. Some of this blue light passesthrough the layer of fluorescent material, and combines with yellowlight emitted by the fluorescent material to provide a white lightoutput. Alternately, the layer of fluorescent material can be omitted sothat the light source emits only the blue light (or another color asdesired) produced by the LED die 171. In either case, the white orcolored light is emitted in essentially a forward direction to producepeak light emission along a symmetry axis of the light source 170. Ifdesired, however, light source 170 can optionally include a deflector177 having reflective surfaces to redirect light in generally sidewaysor lateral directions, thus converting the light source 170 to be aside-emitter. Deflector 177 may have mirror symmetry with respect to aplane perpendicular to the page, or may have rotational symmetry about avertical axis coincident with a symmetry axis of the encapsulating resin176. See also U.S. Pat. 5,959,316 (Lowery).

In FIG. 11, a light source 180 has an LED die 181 supported by a packagebase 182. A lens 183 is coupled to base 182, and a package axis 184passes through the center of base 182 and lens 183. The shape of lens183 defines a volume 184 between LED die 181 and lens 183. The volume184 can be filled and sealed with silicone, or with another suitableagent such as a resin, air or gas, or vacuum. Lens 183 includes asawtooth refractive portion 185 and a total internal reflection (TIR)funnel portion 186. The sawtooth portion is designed to refract and bendlight so that the light exits from lens 183 as close to 90 degrees tothe package axis 184 as possible. See also U.S. Pat. 6,598,998 (West etal.).

In addition to the diverters depicted in FIGS. 8 and 10, the sources canutilize other diverters, including the bifunctional diverters describedin commonly assigned U.S. Application entitled “Direct-Lit BacklightHaving Light Sources With Bifunctional Diverters” (Attorney Docket No.61100US002), filed on even date herewith.

Multicolored light sources, whether or not used to create white light,can take many forms in a backlight, with different effects on color andbrightness uniformity of the backlight output area. In one approach,multiple LED dies (e.g., a red, a green, and a blue light emitting die)are all mounted in close proximity to each other on a lead frame orother substrate, and then encased together in a single encapsulantmaterial to form a single package, which may also include a single lenscomponent. Such a source can be controlled to emit any one of theindividual colors, or all colors simultaneously. In another approach,individually packaged LEDs, with only one LED die and one emitted colorper package, can be clustered together for a given recycling cavity, thecluster containing a combination of packaged LEDs emitting differentcolors such as blue/yellow or red/green/blue. In still another approach,such individually packaged multicolored LEDs can be positioned in one ormore lines, arrays, or other patterns.

Depending on the choice of light source, the back reflector,transflector, and other components of the backlight will be exposed todifferent amounts of UV radiation, with CCFL and HCFL sources emittingmore UV radiation in general than LED sources. Hence, components of thebacklight may incorporate UV absorbers or stabilizers, or may utilizematerials selected to minimize UV degradation. If low UV-output sourcessuch as LEDs are used to illuminate the backlight, UV absorbers and thelike may not be necessary, and a wider selection of materials isavailable. In addition to UV absorbers and stabilizers, the transflectormay also comprise dyes and/or pigments to adjust transmission, color,and other optical characteristics of the transflector, recycling cavity,and backlight.

Reducing color shift is a key problem with LED backlights and generallywill require the use of color sensors and temperature sensors, since LEDoutput can be affected by both of these factors. Color shift occurswhen, for example, the three RGB LEDs exhibit different emissioncharacteristics due to temperature or temporal change. It may benecessary to use a sensor to detect LED fluctuation and automaticallyadjust LEDs or individual LCD pixel intensity to compensate. Sensorlocations could be internal to a recycling cavity if it is desirable tocontrol cavities individually. If individual cavity control is notnecessary, one or more sensors could be located outside the tunnel.Suitable sensors include silicon photodiode color sensors available fromTexas Advanced Optical Solutions Inc (TAOS), Plano Tex.

EXAMPLES Testing Equipment and Setup

The following examples were tested in a custom LED backlight test bed.The test bed was designed to simulate an LED-based area backlight for a559 mm (22″) diagonal, 16:9 aspect-ratio, LCD television. The test bedhad an open rectangular box frame forming side walls of the backlightcavity, the long axis of the frame being placed horizontally. The insidewalls of the box frame were lined with the EDR II film described above,which is a high reflectivity diffuse white film.

The front side of the box frame was covered with a removable diffuserplate made from a diffuse white polymethyl methacrylate plate (CyroCorp., Rockaway, N.J.) about 3 mm thick. This diffuser plate is similarto diffuser plates currently used in CCFL and LED-based televisionbacklights. The outer surface of the plate serves as the output surfacefor the test bed (i.e., the output area of the backlight).

A backplane was attached to the back side of the box frame on fourlinear slides that allowed the backplane to be adjusted to differentdepths within the backlight cavity.

Four LED bars were affixed to the backplane on the side of the backplanefacing the diffuser plate. The bars were arranged in two horizontal rowsspanning the width of the backplane. Each bar had 5 red, 5 blue, and 10green side-emitting Luxeon™ LEDs (Lumileds, San Jose, Calif.) arrangedin a repeating green-red-blue-green pattern in a single line on astandard printed-circuit board. The center-to-center spacing betweenLEDs on a single bar was about 12 mm. The center-to-center spacingbetween adjacent horizontal rows was 152 mm.

On a single bar, the green, red, and blue LEDs were electricallyconnected in series by color so that the output of each color could bevaried independently to allow for adjusting the color balance of thetest bed. Two, two-channel power supplies were connected to each bar.One power supply channel provided the drive current to the red LEDs, onechannel provided current to the blue LEDs, and two channels providedcurrent to the green LEDs each channel driving 5 of the green LEDs.During a typical measurement, the red LEDs were driven at about 150 mA,the blue LEDs were driven at about 170 mA, and the green LEDs weredriven at about 130 mA. Before the first measurements were taken, theLEDs were ‘burned-in’ by running them all at 350 mA for 166 hours, afterwhich the output from the test bed was observed to be relatively stableover time.

A polycarbonate reflector support plate was attached to the backplaneover the LED circuit boards. The reflector support plate was rectangularand slightly smaller than the inside of the test bed frame. Thereflector support plate had holes to allow the LED lenses to extendthrough the plate. When mounted, the top surface of the reflectorsupport plate was aligned with the bottom of the LED lenses. Ahigh-reflectivity, specular back reflector film (Vikuiti™ ESR film from3M) was laminated to the reflector support plate. Thus mounted, the filmlayer was substantially flat and acted as the bottom surface of theoptical cavity of the backlight (i.e., it acted as the back reflector).Directly beneath the back reflector film, the support plate was providedwith linear grooves or channels extending parallel to the rows of LEDs,and disposed on both sides of the LED rows. By slitting the backreflector film immediately above two selected channels (between whichwas at least one row of LEDs), a transflector film whose width wasgreater than the distance between the selected channels could becompressed, forming a concave arc or bow, and opposed edges of thetransflector film inserted into the selected channels through the slitsin the back reflector film. A recycling cavity could thus be formedbetween the back reflector film and the concave-shaped transflector.

The performance of the test bed was measured using a calorimetric camera(model PM 1611 from Radiant Imaging, Inc., Duvall, Wash.). The camerawas fitted with a 105 mm lens and a ND2 neutral density filter. Thesoftware supplied by Radiant Imaging was used to calibrate the cameraand take the measurements. Color and luminance calibration was done withthe aid of a spot radiometer (model PR650 from Photo Research, Inc.,Chatsworth, Calif.). The test bed was placed in the verticalorientation, 4 meters in front of the camera. The test bed was alignedto the camera such that the axis of the camera lens was normal to thediffuser plate and aimed approximately at the center of the test bed.

Backlight constructions were measured by mounting the appropriate films(back reflector and test film) in the test bed and setting the backplaneat the appropriate position to achieve the desired backlight cavitythickness (defined as the space between the top of the back reflectorplate and the bottom of the diffuser plate). Backlight cavitythicknesses that were used include 18, 28, 38, and 48 mm. For recyclingcavities, the films were cut to a specific width and then affixed to thebackplane by wedging the edges of the film into the channels cut in thebackplane, the stiffness of the film causing it to form into an arc andremain in position. The recycling cavity height or depth is determinedby the width of the film and the spacing between the channels that thefilm was wedged into.

The LEDs were turned on and warmed up for at least 90 minutes prior torecording any measurements. Measurements were carried out by configuringthe test bed with the films to be tested, and then using thecolorimetric camera to take pictures of the test bed with the backplaneset at various depths. The results were inspected visually and analyzedfor properties such as total luminance, luminance uniformity, and coloruniformity across the surface of the diffuser plate.

Control

The backlight cavity was configured having no recycling cavity filmsbetween the ESR back reflector and the diffuser plate. The backlightcavity depth was 28 mm from the top of the back reflector to the bottomof the diffuser plate.

The appearance of the output area, i.e. the top of the diffuser plate,was highly non-uniform. An image or blur corresponding to each LED andtheir respective colors was clearly visible at the output area.

Example 1 Two Linear Cavities, W/d=6.7

In this example, the recycling cavities were formed from two layers ofDRPF polarizing film as described above laminated together, the passaxes of the two layers being perpendicular to one another. The backlightcavity was configured with two individual recycling cavities defined bythe ESR back reflector and the layered DRPF films, the cavities takingthe shape of an arc where the top of the arcs were approximately 18 mmabove the back reflector. The width of each recycling cavity wasapproximately 121 mm and they were situated in the backlight cavity suchthat each recycling cavity was approximately 13 mm from the top andbottom sides respectively and were separated in the middle byapproximately 13 mm. The LEDs were oriented horizontally across thebacklight and were parallel to and located within the recyclingcavities. The backlight cavity depth was 28 mm from the top of the backreflector to the bottom of the diffuser plate.

In appearance, this example demonstrated an improvement over the controlwith respect to brightness and color uniformity.

Example 2 Two Linear Cavities, W/d=10.1

In this example, the recycling cavities were formed using a cube cornerlight deflecting film as the transflector. This film was a single layerof 10 mil thick polycarbonate sheeting, the sheeting being flat andsmooth on one major surface and having a pattern of prismatic pits orvoids formed in the opposed major surface. The pattern of prismatic pitswas the inverse or complement of the pyramidal cube corner array used in3M™ Scotchlite™ Reflective Material 6260 High Gloss Film available from3M Company, which pyramidal array is characterized by canted cube cornerprisms whose height (from triangular base to cube corner peak) is about3.5 mils, and whose base triangles have included angles of 55, 55, and70 degrees. This transflective film was oriented such that thestructured or pitted side of the film faced the LEDs. The backlightcavity was configured with two individual recycling cavities defined bythe ESR back reflector and transflector, the cavities taking the shapeof an arc where the top of the arcs were approximately 12 mm above theback reflector. The width of each recycling cavity was approximately 121mm and they were situated in the backlight cavity such that eachrecycling cavity was approximately 13 mm from the top and bottomrespectively and were separated in the middle by approximately 13 mm.The LEDs were oriented horizontally across the backlight and were areparallel to and located within the recycling cavities. The backlightcavity depth was 28 mm from the top of the back reflector to the bottomof the diffuser plate.

In appearance, this example demonstrated an improvement over the controlwith respect to brightness and color uniformity.

Example 3 One Linear Cavity, W/d=11.5

In this example, the recycling cavity was formed from a two layerlaminate of Vikuiti™ BEF II 90/50 linear prismatic optical films,available from 3M Company. A first piece of this BEF II film wasattached to a second piece of BEF II film using adhesive, the linearprisms of the first BEF II film being oriented perpendicular to thelinear prisms of the second BEF II film. As described in U.S. Pat. No.6,846,089 (Stevenson et al.), a thin layer of adhesive was applied tothe planar side of the first film, and the structured side of the secondfilm was placed against the adhesive to form the laminate. The thicknessof the adhesive layer was such that only the prism tips of the secondfilm were immersed in the adhesive layer. The prisms of both pieces ofBEF II faced toward the LEDs, and the prism direction of the BEF II filmnearest the LEDs was parallel to the LED bars. The backlight cavity wasconfigured with a single recycling cavity defined by the ESR backreflector and the BEF II laminate, the recycling cavity taking the shapeof a single arc. The apex of the arc was approximately 22 mm above theback reflector. The width of the recycling cavity was approximately 254mm and it was situated in the backlight cavity such that it wasapproximately 13 mm from the top and bottom respectively. The LEDs wereoriented horizontally across the backlight and were parallel to andlocated within the recycling cavity. The backlight cavity depth was 28mm from the top of the back reflector to the bottom of the diffuserplate.

In appearance, this example demonstrated an improvement over the controlwith respect to brightness and color uniformity.

Example 4 Two Linear Cavities, W/d=6.7

In this example, the recycling cavities were formed from a two layerlaminate of BEF II optical films as described above. A first piece ofBEF II film was attached to a second piece of BEF II film usingadhesive, the linear prisms of the first BEF II film being orientedperpendicular to the linear prisms of the second BEF II film. Asdescribed in U.S. Pat. 6,846,089 (Stevenson et al.), a thin layer ofadhesive was applied to the planar side of the first film and thestructured side of the second film was placed against the adhesive toform the laminate. The thickness of the adhesive layer was such thatonly the prism tips of the second film were immersed in the adhesivelayer. The prisms of both pieces of BEF II faced toward the LEDs, andthe prism direction of the BEF II film nearest the LEDs was parallel tothe LED bar. The backlight cavity was configured with two individualrecycling cavities defined by the ESR back reflector and the BEF IIlaminate taking the shape of arcs where the top of the arcs wereapproximately 18 mm above the back reflector. The width of eachrecycling cavity was approximately 121 mm and they were situated in thebacklight cavity such that each recycling cavity was approximately 13 mmfrom the top and bottom respectively and were separated in the middle byapproximately 13 mm. The LEDs were oriented horizontally and wereparallel to and located within the recycling cavities. The backlightcavity depth was 28 mm from the top of the back reflector to the bottomof the diffuser plate.

In appearance, this example demonstrated an improvement over the controlwith respect to brightness and color uniformity.

Example 5 Two Linear Cavities, W/d=9.5

In this example, the recycling cavities were formed from perforatedVikuiti™ ESR film as described above, which was laminated to apolycarbonate substrate having a slight amount of haze. The perforatedESR film had holes whose diameters ranged from about 0.25 mm to 0.75 mmin a substantially random distribution over the surface of the film, butthe centers of these holes were regularly arranged in a hexagonallattice with hole-to-hole (measured from centers) spacing ofapproximately 1.5 mm. The backlight cavity was configured with twoindividual recycling cavities defined by the ESR back reflector andperforated ESR films, the recycling cavities taking the shape of an arcwhere the top of the arc was approximately 14 mm above the reflectiveback reflector. Each recycling cavity was approximately 133 mm wide andwas situated in the backlight cavity such that each recycling cavity wasapproximately in contact with the top and bottom sidewalls and wereseparated in the middle by approximately 13 mm. The LEDs were orientedhorizontally across the backlight and were parallel to and locatedwithin the recycling cavities. The backlight cavity depth was 28 mm fromthe top of the back reflector to the bottom of the diffuser plate.

In appearance, this example demonstrated an improvement over the controlwith respect to brightness and color uniformity.

Measurement Results

A comparison of Examples 1-5 is shown in Table 1. A Relative Efficiencyparameter was calculated for each backlight construction by dividing theaverage brightness of the example by the average brightness of thecontrol, where average brightness in each case was calculated forsubstantially the entire output area of the respective backlights. ABrightness Non-Uniformity parameter was calculated for each backlightconstruction by dividing the standard deviation of brightness by theaverage brightness over substantially the entire output area of thebacklight. A Color Non-Uniformity parameter (Δuv) was calculated foreach backlight construction as the average value of the point-wisedeviation of the color from the average color of the example, wherecolor is expressed in CIE u′v′ color space. Thus,${{\Delta\quad{uv}} = {\frac{1}{N}{\sum\limits_{N}\sqrt{\left( {u^{\prime} - u_{avg}^{\prime}} \right)^{2} + \left( {v^{\prime} - v_{avg}^{\prime}} \right)^{2}}}}},$

where N is the number of pixels in the image of the test system, u′ andv′ are the color coordinates for each pixel, and U′_(avg) and V′_(avg)are the average color coordinates. This color non-uniformity wasmeasured not over the entire output area, but over a rectangular portionof the output area 10.5 inches long and 3 inches wide, centered over theupper LED bar or row of the backlight prototype. of the entire testsystem. TABLE 1 Relative Efficiency, Brightness Non-Uniformity, ColorNon-Uniformity Relative Brightness Color Example EfficiencyNon-Uniformity Non-uniformity Control 100% 12.5% 0.0090 Example 1 88%11.7% 0.0045 Example 2 104% 7.1% 0.0065 Example 3 91% 9.3% 0.0048Example 4 93% 10.1% 0.0050 Example 5 82% 7.0% 0.0043

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified by the term “about.” Accordingly,unless indicated to the contrary, the numerical parameters set forth inthe present specification and claims are approximations that can varydepending upon the desired properties sought to be obtained by thoseskilled in the art utilizing the teachings disclosed herein.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the scope and spiritof this invention, and it should be understood that this invention isnot limited to the illustrative embodiments set forth herein. All U.S.patents and other patent and non-patent documents referred to herein areincorporated by reference, to the extent they are not inconsistent withthe foregoing disclosure.

1. A direct-lit backlight having an output area, comprising: a backreflector; a transflector that partially transmits and partiallyreflects incident light, the transflector being shaped to form at leastone concave structure facing the back reflector to provide one or morerecycling cavities therebetween, the one or more recycling cavitiessubstantially filling the output area of the backlight; and at least onelight source disposed behind the output area to inject light into theone or more recycling cavities.
 2. A direct-lit backlight having anoutput area, comprising: a back reflector; transflector means forpartially transmitting and partially reflecting incident light, thetransflector means including at least one concave structure facing theback reflector to provide one or more recycling cavities therebetween,the one or more recycling cavities substantially filling the output areaof the backlight; and light source means disposed behind the output areafor injecting light into the one or more recycling cavities.
 3. Thebacklight of claim 1, wherein the at least one light source is disposedin the one or more recycling cavities or disposed behind the backreflector.
 4. The backlight of claim 1, wherein each recycling cavityhas a depth d and a width W, and W is at least 5 times d.
 5. Thebacklight of claim 1, wherein each recycling cavity has a depth d and awidth W, and W is at least 10 times d.
 6. The backlight of claim 1,wherein each recycling cavity is hollow.
 7. The backlight of claim 1,wherein the at least one concave structure consists essentially of asingle concave structure, and the one or more recycling cavitiesconsists essentially of a single recycling cavity.
 8. The backlight ofclaim 1, wherein the at least one concave structure includes a pluralityof concave structures, and the one or more recycling cavities includes aplurality of recycling cavities.
 9. The backlight of claim 8, whereinthe concave structures each have a concave cross-sectional profile in afirst plane and a substantially flat cross-sectional profile in a secondplane perpendicular to the first plane.
 10. The backlight of claim 8,wherein the concave structures each have concave cross-sectionalprofiles in both a first and second mutually perpendicular plane. 11.The backlight of claim 8, wherein each of the recycling cavities extendsacross a dimension of the output area.
 12. The backlight of claim 1,wherein the at least one light source comprises a plurality of LEDs. 13.The backlight of claim 12, wherein the plurality of LEDs comprises LEDsthat emit in different colors.
 14. The backlight of claim 12, whereinthe transflector is shaped to form a plurality of concave structuresfacing the back reflector to provide a plurality of recycling cavities,and wherein for each recycling cavity there is at least one LED disposedbehind the output area to inject light into such recycling cavity. 15.The backlight of claim 1, wherein the transflector consists essentiallyof a structure selected from the group of a semi-reflective film and alight deflecting film.
 16. The backlight of claim 1, wherein thetransflector includes two films selected from the group ofsemi-reflective films, light deflecting films, and combinations thereof.17. The backlight of claim 1, wherein the transflector comprises ascored film.
 18. The backlight of claim 1, wherein the transflectorcomprises a film held in compression.
 19. A display system comprising adisplay panel and the backlight of claim
 1. 20. The system of claim 19,wherein the display panel comprises a liquid crystal display (LCD). 21.The system of claim 20, wherein the system comprises an LCD television.